The objective of this study is to cultivate in vitro P. carinii so as to provide a suitable system and an available resource of organisms for the study and eventual control of infection by this agent in the immunocompetent patient. The growth of P. carinii derived from murine source has been accomplished in chick embryonic lung cells. After three passages a three-fold increase in the number of cyst forms had occurred. Evidence of DNA replication was obtained by autoradiography. Isotopic studies showed that DNA, RNA and protein synthesis of the host cell undergo inhibition during the first 24 hours post infection.